1. Field of the Invention
The present invention relates to a method and apparatus for image forming, and more particularly to a method and apparatus for image forming capable of effectively fixing a toner image on a recording sheet by using induction heating which can effectively and speedily control a temperature of a fixing member.
2. Description of the Background Art
Background image forming apparatuses, such as copiers and printers, include fixing units using an induction heating method. The induction heating method may shorten a time period required for the fixing units to become operable after the fixing units are powered on, and reduce energy consumption.
One example of the fixing units includes a fixing belt, a support roller, an auxiliary fixing roller, an induction heater, and a pressure roller. The fixing belt is laid across the support roller and the auxiliary fixing roller. The induction heater faces the support roller via the fixing belt. The pressure roller faces the auxiliary fixing roller via the fixing belt. The induction heater includes a coil and a core. The coil is wound around the core and extends in directions parallel to a surface of a recording sheet in conveyance and perpendicular to a conveyance direction of the recording sheet which is conveyed between the pressure roller and the auxiliary fixing roller.
A high-frequency alternating current is applied to the coil to generate a magnetic field around the coil. The magnetic field induces an eddy current near a surface of the support roller. Electrical resistance of the support roller generates Joule heat. The fixing belt is heated by the Joule heat via the support roller. The heated fixing belt heats and fixes a toner image on the recording sheet at a position where the pressure roller and the auxiliary fixing roller oppose to each other.
In the above fixing unit, it is possible to increase a surface temperature of the fixing belt to a target fixing temperature in a short time period without consuming much energy.
Another example of the fixing units includes a fixing belt and an induction heater including a core wound by a coil. The core is formed to sandwich the fixing belt. The core faces outer and inner circumferential surfaces of the fixing belt to effectively heat the fixing belt.
Yet another example of the fixing units includes a fixing roller with an induction heater including a core. The core is configured to heat the fixing roller. When a toner image on a small-size recording sheet is fixed, the recording sheet does not contact axis ends of the fixing roller, resulting in increased temperatures of the axis ends of the fixing roller. To solve this problem, Curie points of the core are adjusted. Curie points of the axis ends of the core are set to be lower than a Curie point of an axis center of the core.
In the above fixing units, however, a temperature of a part of the fixing belt or the fixing roller may overly increase when the image fixing is conducted a number of times in a consecutive manner relative to smaller-size recording sheets. Also, a temperature of the whole fixing belt or fixing roller may overly increase when the fixing unit accidentally stops operating due to paper jam.